Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, metal sputtering, plasma oxide deposition, and plasma etching that have been developed for the fabrication of integrated circuits. Digital micromirror devices (DMDs), sometimes referred to as deformable mirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator which uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs are typically operated in a digital bistable mode of operation and as such are the core of true digital full-color image projection systems.
Many different kinds of micromirror devices exist, including torsion beam devices, and hidden-hinge devices. All micromirror devices, however, are usually operated in one of two modes of operation. The first mode of operation is an analog mode, sometimes called beam steering, wherein the address electrode is charged to a voltage corresponding to the desired deflection of the mirror. Light striking the micromirror device is reflected by the mirror at an angle determined by the deflection of the mirror. Depending on the voltage applied to the address electrode, the cone of light reflected by an individual mirror is directed to fall outside the aperture of a projection lens, partially within the aperture, or completely within the aperture of the lens. The reflected light is focused by the lens onto an image plane, with each individual mirror corresponding to a pixel on the image plane. As the cone of reflected light is moved from completely within the aperture to completely outside the aperture, the image location corresponding to the mirror dims, creating continuous brightness levels.
The second mode of operation is a digital mode. When operated digitally, each micromirror is fully deflected in either of the two directions about the torsion hinge axis. Digital operation uses a relatively large address voltage to ensure the mirror is fully deflected. The address electrodes are driven using standard logic voltage levels and a bias voltage, typically a positive voltage, is applied to the mirror metal layer to control the voltage difference between the address electrodes and the mirrors. Use of a sufficiently large mirror bias voltage, a voltage above what is termed the threshold voltage of the device, ensures the mirror will fully deflect toward the address electrodexe2x80x94even in the absence of an address voltage. The use of a large mirror bias voltage enables the use of low address voltages since the address voltages need only slightly deflect the mirror prior to the application of the large mirror bias voltage.
To create an image using the micromirror device, the light source is positioned at an angle relative to the device normal that is twice the angle of rotation so that mirrors rotated toward the light source reflect light in a direction normal to the surface of the micromirror device and into the aperture of a projection lensxe2x80x94creating a bright pixel on the image plane. Mirrors rotated away from the light source reflect light away from the projection lensxe2x80x94leaving the corresponding pixel dark. Intermediate brightness levels are created by pulse width modulation techniques in which the mirror rapidly is rotated on and off to vary the quantity of light reaching the image plane. The human eye integrates the light pulses and the brain perceives a flicker-free intermediate brightness level.
As can be appreciated, the angle at which a micromirror tilts in response to an applied voltage is an important characteristic of a micromirror. Indeed, the tilt angle of a micromirror must be uniform over the entire surface of a micromirror array in order for the array to effectively project images on a display screen. Thus, to properly characterize a micromirror array device, a tilt angle should be measured for the device based upon a given applied voltage.
Because the individual mirrors on a micromirror array are too small for mirror tilt angle to be directly measured in an efficient manner, the tilt angle is measured indirectly. Typically, this involves bouncing a light beam off of the micromirror array and measuring the angle at which the light beam is reflected. This process can become complicated due to the interference and distortion effects created by the wave nature of light. In an attempt to overcome these problems, two methods for measuring tilt angle have been developed: a coherent light measurement system; and a non-coherent light measurement system. Both of these methods, however, have certain limitations and drawbacks.
A coherent light measurement system 100 is depicted in FIG. 1. In FIG. 1, a coherent light source, such as laser 105, generates a coherent light beam 110 that is passed through an aperture 115 in a screen 120. The light beam 110 is then reflected off the surface of a micromirror array 125 into reflected beams 130 that are directed onto a screen 120. Because the micromirror array is a periodic reflecting structure, a diffraction pattern will be generated by the surface of the micromirror array. An example of such a diffraction pattern is illustrated in FIG. 2. In FIG. 2, a series of spots 200 are depicted in a pattern corresponding to the diffraction pattern generated by the micromirror array 125. The pattern includes a first spot 205 corresponding to a beam of light that was reflected from the surface of the micromirror array 125 at an angle equal to the angle of incidence of the incoming light beam 110. Accordingly, the first spot 205 corresponds to a xe2x80x9czeroth orderxe2x80x9d diffraction point. Also depicted in FIG. 2 are a series of spots, 210, 215, 220, 230 and 235, that are arranged on a line that passes through the first spot 205. Each of the spots 210-235 corresponds to increasing diffraction orders, respectively. For example, spot 210 corresponds to a first diffraction order, spot 215 corresponds to a second diffraction order, etc. Furthermore, each of these spots 210-235 are angularly separated by intervals of approximately xcex/T, where xcex is the wavelength of the light beam 110 and T is the mirror pitch/spacing. A plurality of other spots 260 corresponding to the diffraction pattern are also depicted in FIG. 2. It should also be noted that the diffraction pattern 200 is symmetrical about the first spot 205, thereby creating a set of spots on the right-hand side of the pattern that correspond to each of the previously described spots 210-260.
As the mirrors of the micromirror array 125 of FIG. 1 are tilted to different angles, the relative intensity (but not the position) of these spots change. Indeed, in FIG. 2, spot 235 is depicted as having a greater intensity than the other spots. Similarly, spots 240-255 are depicted as having greater intensity than most of the other spots. Thus, to measure the mirror tilt angle, a location along the line of spots 210-235 must be identified that corresponds to the position of greatest intensity of the spots. This measurement process is problematic because it represents only an approximation of the location of greatest intensity, rather than a direct measurement. Furthermore, the use of interpolation (by examining the relative intensity of several adjacent orders and fitting to theoretical distribution) is complicated by the fact that the interpolation is non-linear. These limitations prevent the coherent light measurement system 100 from obtaining measurements of tilt angle with a high level of accuracy.
The invention relates to an improved method and apparatus for measuring the tilt angle of micromirrors arranged in an array. The method and system directs a beam of coherent light through an aperture and onto a micromirror array so that it can be reflected by the array onto a screen. The individual mirrors in the micromirror array are activated and deactivated in a spatial pattern that has a flat power spectrum density distribution, such as a random, aperiodic pattern. Other patterns are suitable for use with this invention, however. The use of a pattern having a uniform power spectrum density reduces the discrete nature of the diffraction pattern that is generated by the micromirror array and thereby generates a pair of spatially distributed (i.e. nondiscrete) intensity patterns on the reflection screen. In an embodiment in which rectangular mirror are used to form the DMD array, the spatially distributed intensity patterns take the general form of a [sin(x)/x]2[sin(y)/y]2 distribution. For the sake of brevity, however, these patterns will be referred to henceforth as a [sin(x)/x]2 distribution. It should be noted that the general method disclosed in this application is valid for measuring mirror tilt angle even if the aperture transform is not a [sin(x)/x]2 function, so long as a spatial pattern is made to appear.
These [sin(x)/x]2 patterns are useful for accurately measuring many characteristics of the micromirror array, such as the tilt angle of the mirror. Although the periodic nature of the micromirror array is greatly reduced through the use of array patterns having uniform power spectral density, some residual periodicity of the array remains. This results in a residual diffraction pattern that can still be seen along with the [sin(x)/x]2 patterns on the reflection screen. By measuring the location of the [sin(x)/x]2 patterns with respect to the residual diffraction pattern, the tilt angle of the micromirror array can be measured with a high level of accuracy. Other characteristics of the micromirror array can also be measured by utilizing this system, such as the xe2x80x9crollxe2x80x9d of the mirrors as they are tilted as well as the tilt angle for xe2x80x9conxe2x80x9d mirrors and xe2x80x9coffxe2x80x9d mirrors separately.